How to " no-flood-fill " a texture?
Use the Fill Tool in the 2d Image Editor and you can then use the Thresh hold setting to dial in your desired tolerance. This threshold feature is not possible as of yet in the 3D view in Texture Paint mode, as it is a different process for painting under the hood. I believe that google search should have brought you to a thread here if I am not mistaken(I made that picture).